The present invention relates to a radio communication system and further relates to a secondary station for use in such a system and to a method of operating such a system. While the present specification describes a system with particular reference to the emerging Universal Mobile Telecommunication System (UMTS), it is to be understood that such techniques are equally applicable to use in other mobile radio systems.
There are two basic types of communication required between a Base Station (BS) and a Mobile Station (MS) in a radio communication system. The first is user traffic, for example speech or packet data. The second is control information, required to set and monitor various parameters of the transmission channel to enable the BS and MS to exchange the required user traffic.
In many communication systems one of the functions of the control information is to enable power control. Power control of signals transmitted to the BS from a MS is required so that the BS receives signals from each different MS at approximately the same power level, while minimising the transmission power required by each MS. Power control of signals transmitted by the BS to a MS is required so that the MS receives signals from the BS with a low error rate while minimising transmission power, to reduce interference with other cells and radio systems. In a two-way radio communication system power control may be operated in a closed or open loop manner. In a closed loop system the MS determines the required changes in the power of transmissions from the BS and signals these changes to the BS, and vice versa. In an open loop system, which may be used in a TDD system, the MS measures the received signal from the BS and uses this measurement to determine the required changes in the transmission power.
An example of a combined time and frequency division multiple access system employing power control is the Global System for Mobile communication (GSM), where the transmission power of both BS and MS transmitters is controlled in steps of 2 dB. Similarly, implementation of power control in a system employing spread spectrum Code Division Multiple Access (CDMA) techniques is disclosed in U.S. Pat. No. 5,056,109.
In considering closed loop power control it can be shown that for any given channel condition there is an optimum power control step size which minimises the Eb/N0 (energy per bit/noise density) required to obtain a particular bit error rate. When the channel changes very slowly the optimum step size can be less than 1 dB, since such values are sufficient to track changes in the channel while giving minimal tracking error. As the Doppler frequency increases (typically but not solely because of the motion of the MS), larger step sizes give better performance, with optimum values reaching more than 2 dB. However, as the Doppler frequency is further increased there comes a point where the latency (or update rate) of the power control loop becomes too great to track the channel properly and the optimum step size reduces again, perhaps to less than 0.5 dB. This is because the fast channel changes cannot be tracked so all that is needed is the ability to follow shadowing, which is typically a slow process.
Because the optimum power control step size can change dynamically it may improve performance if the BS instructs the MS which value of power control step size it should use in uplink transmissions to the BS. An example of a system which uses such a method is the UMTS Frequency Division Duplex (FDD) standard, where power control is important because of the use of CDMA techniques.
A problem in a communication system having variable power control step sizes is how to ensure that the step size remains set to its optimum value. Although the optimum step size for a particular MS speed is known, a MS does not generally know its own speed. Further, the speed of the MS itself is not in practice the only factor affecting the optimum power control step size.
An object of the present invention is to address the problem of dynamically selecting the optimum power control step size.
According to a first aspect of the present invention there is provided a radio communication system having a communication channel between a primary station and a secondary station for transmission of information from one of the primary and secondary stations (the transmitting station) to the other station (the receiving station), wherein the transmitting station has means for adjusting its output power at a plurality of different rates, the receiving station has means for determining, from measurements of characteristics of signals received from the transmitting station, an appropriate rate of adjustment of the output power of the transmitting station and means for communicating said rate of adjustment to the transmitting station, and the transmitting station has means responsive to communications from the receiving station for setting the adjustment rate of its output power.
Different rates of adjustment of output power can be achieved by altering the output power at predetermined intervals by steps of different sizes, or by altering the output power at varying intervals by steps of a predetermined size, or some combination of the two techniques. Small power control step sizes may be emulated, for example by only changing the output power when a certain number of identical power control commands have been received. The output power may also be varied continuously without steps.
According to a second aspect of the present invention there is provided a primary station for use in a radio communication system having a communication channel between the primary station and a secondary station, wherein means are provided for determining, from measurements of characteristics of signals received from the secondary station, an appropriate rate of adjustment of the output power of the secondary station, selected from one of a plurality of rates of adjustment available to the secondary station, and for communicating said rate of adjustment to the secondary station.
According to a third aspect of the present invention there is provided a secondary station for use in a radio communication system having a communication channel between the secondary station and a primary station, wherein means are provided for determining, from measurements of characteristics of signals received from the primary station, an appropriate rate of adjustment of the output power of the primary station, selected from one of a plurality of rates of adjustment available to the primary station, and for communicating said rate of adjustment to the primary station.
According to a fourth aspect of the present invention there is provided a method of operating a radio communication system having a communication channel between a primary station and a secondary station for transmission of information from one of the primary and secondary stations (the transmitting station) to the other station (the receiving station), the method comprising the receiving station determining, from measurements of characteristics of signals received from the transmitting station, an appropriate rate of adjustment of the output power of the transmitting station, selected from one of a plurality of rates of adjustment available to the transmitting station, and communicating the determined rate of adjustment to the transmitting station, and in response the transmitting station setting the adjustment rate of its output power.
The present invention is based upon the recognition, not present in the prior art, that the optimum power control step size can be determined from characteristics of received signals.